In internal combustion engines, the bearing assemblies typically each comprise a pair of half bearings retaining a crankshaft that is rotatable about an axis. Each half bearing comprises a hollow generally semi-cylindrical bearing shell, and typically at least one is a flange half bearing, in which the bearing shell is provided with a generally semi-annular thrust washer extending outwardly (radially) at each axial end. In some half bearings, a single-piece construction of the bearing shell and thrust washers is used, whilst in other half bearings, the bearing shell and the thrust washer are loosely mechanically engaged with clip-like features, and in a further type of half bearing the thrust washers are permanently assembled onto the bearing shell by deformation of engagement features. In other bearing assemblies it is also known to use annular or circular thrust washer.
The bearing surfaces of sliding bearings generally have a layered construction. The layered construction frequently comprises a strong backing material, such as steel, of a thickness in the region of about 1 mm or more; a layer of a first bearing material (the “lining layer”), such as a copper-based material (e.g. copper-tin bronze) or aluminium-based material, is adhered to the backing, and of a thickness generally in the range from about 0.1 to 0.5 mm (e.g. 300 μm of copper-based alloy of 8% wt Sn, 1% wt Ni, and balance of Cu, apart from incidentally impurities); and a layer of a second bearing material (the “overlay layer”) of a metallic or polymer-based bearing material adhered to the surface of the lining layer and having a thickness of less than about 25 μm.
The surface of the second layer forms the actual running or sliding surface, which, in use, faces the surface of a co-operating member, e.g. crankshaft journal. In the case of a bearing shell, the backing provides strength and resistance to deformation of the bearing shell when it is assembled in a main bearing housing or in a connecting rod big end, for example. The first layer may provide suitable bearing running properties, if the second layer should become worn through. Whilst the first bearing material provides seizure resistance and compatibility, it is generally harder than the material of the second layer. Thus, the first layer is commonly inferior to the second layer in terms of its ability to accommodate small misalignments between the bearing surface and the shaft journal (conformability) and in the ability to embed dirt particles circulating in the lubricating oil supply, so as to prevent scoring or damage to the journal surface by the debris.
The first bearing material is commonly chosen from either an aluminium-based alloy (i.e. having no more than 25% wt additive elements, with the balance to 100% wt of aluminium, apart from incidental impurities) or a copper-based alloy material (i.e. having no more than 20% wt additive elements, with the balance to 100% wt of copper, apart from incidental impurities). Aluminium-based alloys generally comprise an aluminium alloy matrix having a second phase of a soft metal therein. Generally, the soft metal phase may be chosen from one or more of lead, tin and bismuth, however, lead is nowadays a non-preferred element due to its environmental disadvantages. Copper-based alloys such as copper-lead and leaded bronzes are also likely to fall into disfavour eventually due to these environmental considerations and may be replaced by lead-free copper alloys, for example.
The second bearing material layer, which forms a mating fit with the shaft journal with a clearance for lubricating fluid, is also known as an overlay layer and is formed of a matrix of plastics polymer material, typically comprising solid filler particulate, and for example has a thickness of 4 to 40 μm.
W02010066396 describes a related plastics polymer-based composite bearing layer comprising a matrix of a polyimide/amide plastics polymer material, and having distributed throughout the matrix: from 5 to less than 15% vol of a metal powder; from 1 to 15% vol of a fluoropolymer particulate, the balance being the polyimide/amide resin apart from incidental impurities (e.g. a layer of 12 μm thickness that comprises 12.5% vol Al, 5.7% vol PTFE particulate, 4.8% vol silane, <0.1% vol other components, and balance (approximately 77% vol) polyimide/amide). Although the compositions of the plastics polymer-based bearing layer of W02010066396 provides high wear resistance and fatigue strength, it remains desirable to further increase the seizure resistance of such polymer-based layers in sliding bearings, particularly in the overlay layers of such sliding bearings.
Fuel-saving operating schemes have become popular for automotive engines, which increase the frequency with which the engine is started. Under a “stop-start” operating scheme, stopping and restarting vehicle movement also leads to the engine being stopped and restarted. Under a “hybrid” operating scheme, the engine is turned off when the vehicle can be powered by an alternative power source, commonly being electrically powered. The greater frequency with which the engine is started under such operating schemes places an increased demand upon the performance of the thrust washers and bearing shells by increasing the frequency with which the counterface of the associated web and journals of the crankshaft respectively contact the thrust washers and bearing shells, and cause correspondingly increased wear of the running surfaces.
In piston assemblies, it is known to reduce the likelihood of scuffing between the piston skirt 10 and cylinder wall, it is well known to provide lubrication between the pistons and cylinder walls from the lubrication oil in the crankcase. In addition, it is known to provide engine components with wear resistant coatings. For example, it is known to coat piston cylinder liners with Nikasil®, which is a electrodeposited lipophilic nickel matrix silicon carbide coating. Similarly, it is known to coat the piston skirt with a material to assist in lubrication and to avoid metal-to-metal contact between the two components, thereby reducing wear, improving lubrication, and/or thermal properties within the cylinder, as described in W02005042953.
Four-stroke engines commonly use three piston rings on each piston, being two compression rings and one oil ring. The role of the compression rings is to prevent the passage of the combustion gases into the crankcase, whereas the function of the oil ring is to scrape the excess oil from the cylinder wall and return it to the crankcase, controlling the thickness of the ‘film’ of oil and preventing it from being unduly burned. Another important function of the rings is to act as a bridge to transmit heat from the piston to the cylinder wall or cylinder liner (also known as a cylinder sleeve), where it is dissipated by way of the cooling system. On the piston rings, in particular on the compression rings, it is known to apply a coating layer on at least the outer circumference that comes into contact with the cylinder wall, as is described in W02011000068.